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Agenesis of the Corpus Callosum

Interhemispheric cyst (*) communicating with widely splayed frontal horns (arrows)

Dilated, displaced "steer-horn" ventricles (yellow arrows). Absent corpus callosum (red arrowheads)

Colpocephaly-Dilated tear-drop configuration of the occipital horn

Findings:

A large interhemispheric cyst (*) is seen communicating with dilated and splayed frontal horns. In addition, there is absence of the corpus callosum in this coronal plane of section. The occipital horns are dilated and have a "tear-drop" configuration.

Discussion:

The corpus callosum is a thick band of commisural fibers connecting the two cerebral hemispheres. It first begins to develop at approximately 10-12 weeks of gestation and is complete at approximately 17-20 weeks. The corpus callosum initially forms anteriorly as the rostrum, followed posteriorly by the genu, body and splenium. Agenesis of the corpus callosum was first described in 1934 by Davidoff and Dyke. It has a prevalance of 0.3% in the general population and in 2-3% of those that are developmentally disabled. Partial agenesis involving the anterior portion of the corpus callosum is most often on the basis of a destructive event ( vascular or infectious) whereas partial agenesis involving the splenium is often due to maldevelopment. While agenesis of the corpus callosum can occur as an isolated anomaly, in one study 85% had concurrent central nervous system abnormalities, many of which were characterized by mental retardation and spasticity. These CNS abnormalities may include midline abnormalities such as the Arnold-Chiari malformation or the Dandy-Walker malformation or destructive lesions such as porencephalic or interhemispheric cysts. 62% had malformations of other organ systems. 10-20% of cases have an abnormal karyotype. When the agenesis is an isolated abnormality, particularly in males, the outcome is often, but not always, normal. In female fetuses, agenesis of the corpus callosum may be the only finding of the Aicardi syndrome.

Genetic Syndromes Associated with Agenesis of the Corpus Callosum

Syndrome
Inheritance Pattern
Other Sonographic Findings

Aicardi

x-linked recessive (females)

None

Andermann

Autosomal recessive

None

Acrocallosal

Autosomal recessive

Polydactyly, cerebellar hypoplasia, hypertelorism

Lissencephaly

Autosomal recessive or in association with17p13 deletion

Agyria, urogenital abnormalities, cardiac abnormalities, IUGR,polyhydramnios, microcephaly

Shapiro

Autosomal recessive

None

Apert

Autosomal dominant

Craniosynostosis, syndactyly, frontal encephalocele

Rubenstein-Tabyi

16p13.3 deletion

Stubb fingers, cardiac and renal abnormalities

Oral-facial-digital I

x-linked dominant

Cleft palate, asymmetric shortening of digits with clinodactyly with or without syndactyly

Frontonasal dysplasia

Unknown

Hypertelorism, cleft lip, cleft nose

From Vergani P, Ghidini A, Strobelt N, Locatelli A, Mariani S, Bertalero C, Cavallone M. Prognostic indicators in the prenatal diagnosis of agenesis of the corpus callosum. Am J Obstet Gynceol 170:753-758, 1994

The initial signs of agenesis of the corpus callosum were first described based upon findings at pneumoencephalography. Antenatal ultrasound can often be utilized in attempting to identify fetuses with this condition. As was mentioned above, the corpus callosum is not fully formed until the middle of the second trimester. Abnormalities are often not diagnosed until the end of the 2nd trimester or 3rd trimester. The corpus callosum is best seen sonographically on midline sagittal or coronal frontal planes of section. These planes of section may not be easily obtained especially when the fetus is in cephalic presentation. As the corpus callosum itself is often difficult to identify, secondary signs which are dependent on changes of the cerebral hemispheres and ventricular system must be relied upon. The cavum septum pellucidi requires the corpus callosum to develop normally for this stucture to also form normally. Absence of the cavum septum pellucidum may be a clue that this abnormality is present. Its presence exludes this condition. It should be remembered that the cavum septum pellucidi may be absent in other pathological conditions such as holoprosencephaly. Three other signs rely upon abnormalities of the ventricular system: 1) The occipital horn is often enlarged with a "tear-drop" configuration (narrow end anteriorly) and the lateral ventricular wall are often parallel instead of diverging laterally.The enlargement and configuation of the occipital horn is referred to as colpocephaly. It should be remembered that disproportionate enlargement of the occipital horns will also be seen with hydrocephalus from many causes (although without the "teardrop configuration". 2) The frontal horns which may be dilated are widely separated in what has been referred to as "steer-horn" vetricles. 3) the third ventricle is often dilated and elevated, appearing as a large midline cyst. As stated above, interhemispheric cysts are often seen in this condition and may be the initial finding that leads one to the correct diagnosis.

References:

Bennett GL, Bromley B, Benacerraf BR. Agenesis of the corpus callosum: prenatal detection usually is not possible before 22 weeks gestation. Radiology 199:447-450, 1996

Hilpert PL, Kurtz AB. Prenatal diagnosis of agenesis of the corpus callosum using endovaginal ultrasound. J Ultrasound Med 9:363-365, 1990

Vergani P, Ghidini A, Strobelt N, Locatelli A et al. Prognostic indicators in the prenatal diagnosis of agenesis of corpus callosum. Am J Obstet Gynecol 170:753-758, 1994

Comstock CH, Culp D, Gonzalez J, Boal DB. Agenesis of the corpus callosum in the fetus: its evolution and significance. J Ultrasound Med 4, 613-616, 1985

Parrish M, Roessman U, Levinsohn M. Agenesis of the corpus callosum: a study of the frequency of associated malformations. Ann Neurol 6:349, 1979

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Peter W. Callen, M.D.
Professor of Radiology, Obstetrics, Gynecology and Reproductive Science
University of California Medical Center, San Francisco, California